A major product in the acid-catalyzed dehydration of fructose is 2-hydroxymethyl-5-furfuraldehyde, also known as hydroxymethylfurfural (HMF).
HMF represents one key intermediate substance readily accessible from renewable resources like carbohydrates and is a suitable starting source for the formation of various furan monomers which are used for the preparation of non-petroleum-derived polymeric materials. HMF and its principle derivative, furandicarboxylic acid (FDCA) are molecules having an enormous potential, in particular for the production of polymers, in particular polyamides or polyesters, because of their structural similarities with terephthalic acid, a monomer usually used. While not being bound by theory, it is generally believed that fructose is converted to HMF via an acyclic pathway, although evidence also exists for the conversion to HMF via cyclic fructofuransyl intermediate pathways. Regardless of the mechanism of HMF formation, the intermediate species formed during the reaction may in turn undergo further reactions such as condensation, rehydration, reversion and other rearrangements, resulting in a plethora of unwanted side products. HMF can be obtained by dehydration, in an aqueous or solvent medium, of carbohydrates, in particular fructose, glucose or cellulosic material. However, the conversion and selectivity (and ultimately the yield) are low, in particular in purely aqueous or solvent media.
The lack of HMF selectivity of the carbohydrate dehydration reaction is explained by the rapidity of the secondary polymerization reactions of the reaction intermediates or of the HMF in a purely aqueous or solvent medium with an acid catalyst (for example, the formation of char).
Although preparation of HMF has been known for many years, a method which provides HMF with good selectivity and in high yields has yet to be found. Complications arise from the hydrolysis of HMF, which yields by-products, such as, levulinic and formic acids. Another unwanted side reaction includes the polymerization of HMF and/or fructose resulting in humin polymers, which are solid waste products. Further complications may arise as a result of solvent selection. Water is a relatively inexpensive solvent and dissolves fructose, but unfortunately, low selectivity and increased formation of polymers and humin increases under aqueous conditions.
Thus the selective production of HMF is complex, and its purification is difficult due to the instability of this molecule. It is also difficult to obtain HMF inexpensively. For these reasons, there is still no industrial-scale production of HMF.
Levulinic acid can be used to make resins, plasticizers, specialty chemicals, herbicides and as a flavor substance. Levulinic acid is useful as a solvent, and as a starting material in the preparation of a variety of industrial and pharmaceutical compounds such as diphenolic acid (useful as a component of protective and decorative finishes), calcium levulinate (a form of calcium for intravenous injection used for calcium replenishment and for treating hypocalcemia. The use of the sodium salt of levulinic acid as a replacement for ethylene glycols as an antifreeze has also been proposed.
Esters of levulinic acid are known to be useful as plasticizers and solvents, and have been suggested as fuel additives. Acid catalyzed dehydration of levulinic acid yields alpha-angelica lactone, which may be useful to make polymers.
Levulinic acid has been synthesized by a variety of chemical methods. But levulinic acid has not attained much commercial significance due in part to the high cost of the raw materials needed for synthesis. Another reason is the low yields of levulinic acid obtained from most synthetic methods. Yet, another reason is the formation of a formic acid byproduct during synthesis and its separation from the levulinic acid. Yet, still another reason is the formation of solid char by-products which are costly to remove from the process and need to be disposed, sold, or incinerated. Therefore, the production of levulinic acid has had high associated equipment costs. Despite the inherent problems in the production of levulinic acid, however, the reactive nature of levulinic acid makes it an ideal intermediate leading to the production of numerous useful derivatives.